Measured hematocrit:Does it reflect measured red cell volume?

Abstract

Introduction: Hematocrit is a ratio of red cell volume to plasma volume and has been used as a surrogate marker for red cell volume as well as to determine blood transfusions. Patients may tolerate lower hematocrits if the plasma volume is normal, while hypovolemic anemia may lead to poor outcome. It is unclear whether measured hematocrit reflects red cell volume without information regarding plasma volume. Technique for measurement of plasma volume and RBC mass has been cumbersome until improved technology has made it clinically useful. This study was done in surgical intensive care unit patients where hematocrit was compared to a corrected hematocrit calculated from blood volume measurement results using a blood volume analyzer (BVA-100, Daxor, New York). Methods: Consecutive surgical ICU patients whose volume status was deemed uncertain by the treating team had blood volume measurements performed utilizing 5 blood sample draws after albumin I-131 radioisotope injection. Plasma volume was measured and the red cell volume was calculated from the measured hematocrit. A corrected hematocrit was calculated from the measured volume. Corrected hematocrit is defined as what the hematocrit would be if the patient had normal whole blood volume. This study compared measured hematocrit to corrected hematocrit using the Bland and Altmann analysis. Results: 42 patients completed the study with 84 simultaneous blood volume and hematocrit measurements. Average age was 61 ± 20 years with 7 females:35 males. APACHE II score was 20 ± 6 and the mortality rate was 5/42 (12%). There were 15 trauma victims, 21 General Surgery patients, and 6 from other surgical subspecialties with the following problems: 12 with septic shock/severe sepsis, 8 with hemorrhaghic shock, 22 with respiratory failure. The measured hematocrit was lower than the corrected hematocrit in 38 instances (45%), higher in 17 readings (29%) and equivalent (defined as ±3% of each other) in 28 (33%) measurements. The Bland and Altmann analysis showed a mean difference between measurements (bias) of 3.4 ± 7.8 hematocrit percentage points with a 95% Confidence Interval of 1.7 to 5.1. The precision (error) was 15.2 hematocrit percentage points. Conclusion: Measured hematocrit may not reflect true red cell volume in the surgic al ICU patients and may be incorrect as much as 15 percentage points. There is a sub-set of patients who are hypovolemic. Such patients, when their hematocrit is “adjusted or normalized” to normal volume, are significantly more anemic than evidenced by their peripheral hematocrit. Contrary wise, there is a significant percentage of patients who are hypervolemic and whose hematocrit when normalized to true volume status, are shown to be not anemic and who should not receive transfusions. There are patients who should receive transfusions who can only be recognized by blood volume measurement, specifically red cell volume measurement, and another group of patients who are volume-expanded and hemodiluted and who may receive inappropriate transfusions.